The use of water cooling during the continuous casting of steel and aluminum alloys
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I. INTRODUCTION
CONTINUOUS casting processes for both steel and aluminum alloys were developed several decades ago to produce shapes for subsequent semifabrication processes such as extrusion or rolling. As-cast product shapes include billets (square cross section with thickness less than 150 to 175 mm for steel), thick slabs/ingots (wide rectangular cross section with thickness between 50 and 300 mm for steel, and up to 500 to 750 mm for aluminum alloys), thin slabs (thickness between 50 and 75 mm for steel), strips (thickness between 1 and 12 mm for both steel and aluminum alloys), and rounds/extrusion billets (100- to 500-mm diameter for both steel and aluminum alloys). In recent decades, a dramatic growth of this primary metal processing technology has been realized in both steel and aluminum industries, owing to a substantial increase in yield, energy savings, and productivity over static casting. However, the technological advancement has taken distinctly different routes for these two metal industries. Over the years, the casting procedures for steel and aluminum alloy products have developed distinctive features in terms of casting practices, machinery, and process and quality control methodologies. The productivity of both processes is controlled by the casting speed, so higher speeds are always sought. However, the casting speed cannot be increased arbitrarily for several reasons.[1] First, the resulting increase in depth of the liquid pool and surface temperature of the strand prolongs the solidificaJ. SENGUPTA, NSERC (Canada) Postdoctoral Fellow, and B.G. THOMAS, W. Grafton & L.B. Wilkins Professor, are with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801. Contact e-mail: [email protected] M.A. WELLS, Assistant Professor, is with the Department of Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Manuscript submitted May 10, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
tion process and increases the cooling requirements. In extreme cases, the structurally weak solid shell may rupture, leading to a “breakout” of liquid metal below the mold, or to excessive bulging if containment is exceeded for larger sections. Second, higher casting speeds often lead to cracks, caused by the higher thermal stresses. The practical range of operating speeds depends on alloy composition and product geometry. For steel slabs, the casting speed increases with decreasing thickness from 0.01 ms1 (for 300-mm blooms) to over 0.08 ms1 (for 50-mm thin slabs). Owing to cracking difficulties during startup, aluminum alloy ingots and billets are cast at much lower speeds, increasing from 0.00075 to 0.001 ms1[2] to steady speeds ranging from 0.001 to 0.003 ms1[3]. The continuous casting machinery is comprised of the mold and secondary water-cooling systems. These are designed to extract superheat from the incoming liquid metal (5 pct of the total heat content in the metal), latent heat of fusion at the solidification front (20 pct
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